I have never owned a loop for any band, not counting the 2
element quad that I ran for about 10 years on 10, 15, and 20
meters. That was by far the best tri-bander I have ever
owned.
That said, the people that I know around here who use a loop
for multiple bands feed it with either 300 or 450 ohm line.
Some of them go through a ballun near the shack, and others
go straight into the tuner with the ladder line.
Mike Duke, K5XU
American Council of Blind Radio Amateurs